By careful observation and experimentation it has been shown conclusively that sunlight rapidly kills bacteria, and that it is only in dampness and darkness that bacteria thrive and multiply.  Although sunlight is essential to the growth of most plants and animals, it retards and prevents the growth of bacteria.  Dirt and dust exposed to the sunlight lose their living bacteria, while in damp cellars and dark corners the bacteria thrive, increasing steadily in number.  For this reason our houses should be kept light and airy; blinds should be raised, even if carpets do fade; it is better that carpets and furniture should fade than that disease-producing bacteria should find a permanent abode within our dwellings.  Kitchens and pantries in particular should be thoroughly lighted.  Bedclothes, rugs, and clothing should be exposed to the sunlight as frequently as possible; there is no better safeguard against bacterial disease than light.  In a sick room sunlight is especially valuable, because it not only kills bacteria, but keeps the air dry, and new bacteria cannot get a start in a dry atmosphere.

CHAPTER XIII

COLOR

126.  The Rainbow.  One of the most beautiful and well-known phenomena in nature is the rainbow, and from time immemorial it has been considered Jehovah’s signal to mankind that the storm is over and that the sunshine will remain.  Practically everyone knows that a rainbow can be seen only when the sun’s rays shine upon a mist of tiny drops of water.  It is these tiny drops which by their refraction and their scattering of light produce the rainbow in the heavens.

The exquisite tints of the rainbow can be seen if we look at an object through a prism or chandelier crystal, and a very simple experiment enables us to produce on the wall of a room the exact colors of the rainbow in all their beauty.

[Illustration:  FIG. 86.—­White light is a mixture of lights of rainbow colors.]

127.  How to produce Rainbow Colors. The Spectrum. If a beam of sunlight is admitted into a dark room through a narrow opening in the shade, and is allowed to fall upon a prism, as shown in Figure 86, a beautiful band of colors will appear on the opposite wall of the room.  The ray of light which entered the room as ordinary sunlight has not only been refracted and bent from its straight path, but it has been spread out into a band of colors similar to those of the rainbow.

Whenever light passes through a prism or lens, it is dispersed or separated into all the colors which it contains, and a band of colors produced in this way is called a spectrum.  If we examine such a spectrum we find the following colors in order, each color imperceptibly fading into the next:  violet, indigo, blue, green, yellow, orange, red.

128.  Sunlight or White Light.  White light or sunlight can be dispersed or separated into the primary colors or rainbow hues, as shown in the preceding Section.  What seems even more wonderful is that these spectral colors can be recombined so as to make white light.